Electromigration degradation mechanism for Pb-free flip-chip micro solder bumps

The electromigration (EM) of flip chip interconnect composed of Pb-free SnAgCu micro solder bump and Ni under bump metallization (UBM) is studied by accelerated current stress test. The process of void growth at the cathode side of the bump and resistance degradation process are investigated by physical analysis using SEM and Infrared microscope. EM degradation process for SnAgCu solder bump shows drastic change over time in degree of resistance shift owing to variation of contact diameter and UBM layer structure. UBM metal diffusion and local Joule heating caused by current crowding play important roles in voiding degradation process.